Wireless communication systems are well known and consist of many types including land mobile radio, cellular radiotelephone (inclusive of analog cellular, digital cellular, personal communication systems (PCS) and wideband digital cellular systems), and other communication system types. In cellular radiotelephone communication systems, for example, a number of communication cells are typically comprised of one or more Base Transceiver Stations (BTS's) coupled to one or more Base Station Controllers (BSCs) or Central Base Station Controllers (CBSCs) and forming a Base Station Subsystem (BSS). The BSCs or CBSCs are, in turn, coupled to a Mobile Switching Center (MSC) which provides a connection between the BSS and an external network, such as a Public Switched Telephone Network (PSTN), as well as interconnection to other BSSs. Each BTS provides communication services to a mobile station (MS) located in a coverage area serviced by the BTS via a communication resource that includes a forward link for transmitting signals to, and a reverse link for receiving signals from, the MS.
Fundamental to a wireless communication system is the ability to maintain established communication connections while an MS moves in and between coverage areas. In order to maintain established communication connections, ‘soft-handoff’ techniques have been developed for code division multiple access (CDMA) communication systems whereby an MS is in concurrent, active communication with multiple BTSs. Each BTS in active communication with the MS is a member of an ‘active set’ of the MS and transmits bearer traffic to, and receives bearer traffic from, the MS. As the MS moves through the communication system, BTSs are added to, or deleted from, the MS's active set so as to assure that the MS will always be in communication with at least one BTS.
For example, FIG. 1 is block diagram of a CDMA wireless communication system 100 of the prior art. Communication system 100 includes a BSS 104 comprising multiple BTSs 106–108 that are each coupled to a CBSC 110. BSS 104 is coupled to an MSC 114 and MSC 114 is in turn coupled to an external network 116 and provides a communication link between the external network, or other BSSs, and BSS 104. Communication system 100 further includes an MS 102 that concurrently is in active communication with each of BTS 106 and 107. That is, MS 102 is in ‘soft-handoff’ with each of BTSs 106 and 107 and each of BTS 106 and BTS 107 is a member of an ‘active set’ of MS 102. As members of the active set of MS 102, each BTS of BTSs 106 and 107 concurrently maintains a respective wireless communication link 120, 130 with the MS. Each communication link 120, 130 includes a respective forward link 122, 132, for conveyance of signals to MS 102 and a respective reverse link 124, 134, for receipt of signals from the MS.
Each BTS 106, 107 in the active set of MS 102 conveys the same bearer traffic to, and receives the same bearer traffic from, the MS. By providing multiple BTSs that concurrently convey same signals to, and receive same signals, from MS 102, communication system 100 enhances the likelihood that the MS will receive an acceptable quality signal from BSS 104 and that the BSS will receive an acceptable quality signal from the MS. For example, when each BTS of BTSs 106 and 107 receives a same frame from MS 102, the BTS determines whether the received frame is acceptable or erroneous and forwards the frame, and an accompanying indicator of whether the frame is acceptable, that is, good, or erroneous, that is, erased, to CBSC 110. CBSC 110, specifically a selection and distribution unit (SDU) 112 included in the CBSC, then selects a version of the received frame from among the versions received from BTSs 106 and 107 and forwards the selected version to MSC 114.
As MS 102 heads towards a coverage area, or sector, associated with BTS 108, a signal strength of a pilot signal received by MS 102 from BTS 108 via a forward link 142 associated with BTS 108 increases until MS 102 identifies the pilot signal from BTS 108 as a viable communication link. In addition, as MS 102 heads away from a coverage area, or sector, associated with BTS 106 a signal strength of a pilot signal received by MS 102 from BTS 106 via forward link 122 may deteriorate to the point that MS 102 determines that communication link 120 is no longer a viable communication link. MS 102 then requests that communication system 100 add BTS 108 to the MS's active set, that is, establish a communication link 140 associated with BTS 108, comprising forward link 142 and a reverse link 144, as an active communication link for transmitting data to, and receiving data from, MS 102, and drop BTS 106 from the active set, that is, terminate communication link 120. Typically, the request to drop a BTS is conveyed by MS 102 via a Pilot Strength Measurement Message (PSMM). Upon receiving the PSMM message, BSS 104 drops BTS 106 from the active set of MS 102 and terminates, or drops, communication link 120 between MS 102 and BTS 106.
A decision to add or drop a communication link 120, 130, 140 and a corresponding BTS 106, 107, 108 from an active set is based on a forward link measurement, that is, a measurement of a signal strength of a pilot signal received by an MS, such as MS 102, via a corresponding forward link. That is, in making an add and/or drop decision, communication system 100 assumes that a performance of a forward link 122, 132, 142 is substantially identical to a performance of a corresponding reverse link 124, 134, and 144. However, a problem arises when there is an imbalance between a performance of a forward link, such as forward links 122, 132, 142, and a performance of a corresponding reverse link, that is, respective reverse links 124, 134, and 144. For example, a sector serviced by a BTS may be subject to interference particular to a reverse link, or propagation paths and fading associated with each of a forward link and a reverse link in a sector may be different due to frequency band separation between the forward and reverse links. As a result, in some cases a communication link with a strong reverse link may be dropped due to a weak forward link while a communication link with a weak reverse link may be added due to a strong forward link. When the reverse link of the dropped communication link, such as reverse link 124 of communication link 120, is the only good reverse link of multiple reverse links, such as reverse links 124 and 134, utilized by an MS while the reverse link of the added communication link, such as reverse link 144 of communication link 140, is of unacceptable quality, then the dropping of the communication link may result in a dropped call.
Thus, a need exists to ensure that a communication link is not terminated, or dropped, as part of a soft-handoff until such time as the communication system may confirm that an acceptable quality reverse link is being added.